How Much Horsepower Does One Man Need? Part Three of Four: Horsepower

By Brad (DangerMouse) Joyce

You have probably used the word horsepower in your life. Horsepower may have even been the deciding factor for you in a recent purchase. The word is thrown around to compare everything from cars and motorcycles to vacuum cleaners and lawn mowers. Can you define what horsepower is, though?

Last month, we discussed torque. For those who seek torque, there is no substitute. But as we discovered, torque doesn't necessarily make your bike faster. In a discussion on the merits of small turbo-charged European engines versus American V-8s, a rather indelicate college professor of mine described the difference between horsepower and torque this way:

"Asking how much torque an engine produces is liking asking how much you have in your pants; asking how many horsepower an engine develops is like asking your girlfriend if she had an orgasm or not."

Colorful guy. Knowing today's colleges, he's probably in jail now. He had a point, though. Torque makes riding a bike fun, but horsepower wins the race.

Scotland is one of two things. It is either a mind-numbingly-boring place, or it is home to a race of geniuses. One way or the other, the final outcome is that a thoroughly disproportionate number of inventors and scientists hail from Scotland. James Watt was one of them. He invented the term horsepower. It is also he who perfected the steam engine. Yes, that's his name on the light-bulb rating, too. The story goes that Watt was apparently fascinated with defining the amount of power one could derive from the ponies that lift coal from mines. Ok, my guess is that Scotland is just boring.

According to his measurements, the average coal pony could lift 1,000 pounds 22 feet in one minute. That's 22,000 foot-pounds a minute. Figuring that a full-sized horse is about 1/3 bigger than these ponies, he guessed that a horse could do about 33,000 foot-pounds of work in a minute. It is this completely arbitrary number that people the world over have accepted as the international standard of measuring power. This should thoroughly confirm for all readers that the world is indeed a very strange place.

A bike with 50 horsepower, therefore, could theoretically move 50 x (33,000 pounds one foot in one minute). That makes 1,650,000 pounds over one foot in a full minute. Well, measuring one foot and one minute at a time are odd ways to describe motorcycles. Let's change the units a bit to clear things up: 50 horses is the ability to move 312.5 pounds one mile in a minute.

Have you noticed something different between torque and horsepower? Torque is measured in terms of weight and distance alone. There is no time component in torque. Horsepower is different. Horsepower has a time element - minutes.

Horsepower is a rate.

For convenience's sake, horsepower can be thought of as torque x RPMs. This is a simplification of the actual formula, but it is sufficient to illustrate the principles involved. In short, an engine's ability to propel you down the road at an illegal rate is primarily determined, therefore, by two things:

1.) Your engine's ability to produce torque, and/or
2.) Your engine's ability to spin RPMs.

"There is no substitute for cubic inches." You may have heard that one. When you're talking cars, is almost always true. Not so in bikes. Different engines are designed to produce different results. I always like to compare the Kawasaki 800 and 1500 Vulcans to illustrate this point. The 1500 has nearly twice the cubic inches of its little brother. The 1500's engine develops nearly twice the torque, too. But the 1500 has a very long stroke. That stroke, while brilliantly producing gobs of delicious torque, does something adverse. Since the pistons have nearly twice the distance to travel as the 800's, it slows the engine's ability to spin its crankshaft. The 800 can spin about twice the RPMs as its big brother. Remember - Horsepower is a product of Torque and RPMs. What does this mean? It means that both engines produce about the SAME amount of horsepower. Above, I said torque and RPMs PRIMARILY determine speed. There is another factor involved, though: weight.

There is no better number to gauge a bike's potential for speed than horsepower divided by weight (including the rider).

The Vulcan 800 weighs considerably less than the 1500. The 1500 feels like a very powerful machine. It's engine throbs proudly in a way that makes riding it a sheer joy. But where the rubber hits the road, the 800 will smoke it. This can potentially disappoint folks spending the extra few thousand to trade their 800's 'up' to the 15. Kawasaki should have been more careful. Harleys are the same. Sportster aficionados are quick to point out that the Sporty 1200 is Harley's fastest bike. Well, stock, that is.

Yamaha didn't make that mistake. The Road Star does maintain a lead on a V Star 650 in a flat out drag. Not as much as you may think, though. Even on full throttle, a Roadie won't leave the little 650 by more than a car length or so. Pipes and a BAK/GAK would probably give the perky 650 a performance edge over a stock Roadie, as a matter of fact. It should be noted that with the Roadie's brutal 98 foot pounds of stock torque, the added weight of a pillion passenger and full bags won't put the tiniest dent in the mighty 1600's performance. Equally, it should be noted that the V Star 1100 would out-drag the Roadie even with its smaller engine.

Yamaha makes different hammers for different nails. There is no substitute for a Roadie if you want a torque-cruiser. Likewise, there is no substitute for the 1100 if you want a performance-cruiser. There is no substitute for the Venture if you want a touring-cruiser. And there is no substitute for the Royal Star if you want a luxury-cruiser. Unlike many manufactures, a Yamaha motorcycle's intended duty is not a function of mere accoutrements bolted on to distinguish a boulevard strutter from a serious tourer. Yamaha carefully designed each bike with a different engine that is perfectly matched to the bike's intended duty.

There is one more important aspect to horsepower. An engine's power is not equal at all RPMs. You have probably noticed that horsepower ratings typically are expressed as a number at a given RPM. 47 @ 5200 RPM, for instance, means that if you plot a line of Horsepower versus RPM for this theoretical engine, the highest horsepower value, 47, occurs at 5200 RPMs. Cruisers have relatively flat horsepower curves. They spike up quickly and stay basically the same until power starts to fade near redline. Other engines aren't necessarily this way. Sport bikes have very 'peaky' curves, which require riders to make constant, skillful shifts in order to maintain maximum power. You have probably done something similar yourself. Have you ever downshifted to get a little more 'umph'? Have you have revved your engine to get better acceleration from a stop? What you are doing is adjusting the engine's RPMs to a higher point in the power curve. This is especially useful on the Royal Star. With a rev-happy four cylinder, the big Royal's power curve is not nearly as flat as the other Stars. Unlike the 650 and the 1600, (and to a lesser extent the 1100) a downshift on a Royal reveals untapped caches of power (especially when modified to maximize performance). When reading magazine articles comparing horsepower ratings, make sure to compare not only the PEAK horsepower rival engines create. Take a look at the power curve. If the curve is flat, the power will always be available with a twist of the throttle. If it is extremely peaky, you may find yourself always hunting for power. An engine with a higher horsepower peak can end up being slower than another bike if the peaky portion of the curve is narrow.

In the September issue of Star Cruiser, look for the final installment in this series -- a discussion about specific methods Star owners can employ to maximize their bike's performance.

To be continued in our next issue
How Much Horsepower Does One Man Need? - Part 3